The strong Fe K line and spin of the black-hole X-ray binary MAXI J1631-479

TL;DR

This study analyzes the black hole binary MAXI J1631-479, revealing that its strong Fe K line is best explained by disk irradiation from Comptonized emission, and highlights the significant model dependence of spin measurements.

Contribution

It demonstrates that the strong Fe K line can be explained by Comptonization irradiation, and shows the spin measurement's dependence on the physical disk model.

Findings

The Fe K line is explained by Comptonization irradiation rather than direct disk emission.

The fitted black hole spin varies significantly depending on the disk model used.

A more realistic disk model yields a high spin of approximately 0.8--0.9.

Abstract

We study the transient black hole binary MAXI J1631--479 in its soft spectral state observed simultaneously by the NICER and NuSTAR instruments. Its puzzling feature is the presence of a strong and broad Fe K line, while the continuum consists of a strong disk blackbody and a very weak power-law tail. The irradiation of the disk by a power-law spectrum fitting the tail is much too weak to account for the strong line. Two solutions were proposed in the past. One invoked an intrinsic Fe K disk emission, and the other invoked disk irradiation by the returning blackbody emission. We instead find that the strong line is naturally explained by the irradiation of the disk by the spectrum from Comptonization of the disk blackbody by coronal relativistic electrons. The shape of the irradiating spectrum at $\lesssim$10 keV reflects that of the disk blackbody; it is strongly curved and has a higher flux than that of a fit with a power-law irradiation. That flux accounts for the line. While this result is independent of the physical model used for the disk intrinsic emission, the value of the fitted spin strongly depends on it. When using a Kerr disk model for a thin disk with a color correction, the fitted spin corresponds to a retrograde disk, unlikely for a Roche-lobe overflow binary. Then, a model accounting for both the disk finite thickness and radiative transfer yields a spin of $a_*\approx0.8$--0.9, which underlines the strong model-dependence of X-ray spin measurements.